Today's Veterinary Practice

SEP-OCT 2018

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todaysveterinarypractice.com SEPTEMBER/OCTOBER 2018 47 FEATURES physiology because many of these cases may go underdiagnosed due to lack of azotemia and lower urinary tract signs. Once patency of the urinary tract is restored, either surgically or via urinary catheterization or stent placement, the clinician should be aware of and closely monitor the effects of postobstructive diuresis on serum potassium and magnesium. Lack of effective monitoring of urine output and electrolytes may lead to hypokalemia and worsening renal function due to intravascular volume depletion. MAGNESIUM Magnesium is a cation that has garnered increasing attention in both human and veterinary medicine over the past 50 years. 15 In the 1950s, Vitale et al. discovered a link between magnesium deficiency and uncoupling of oxidative phosphorylation. Since then, magnesium has been recognized as essential to many metabolic functions, including via its role as a coenzyme for the membrane-bound sodium-potassium adenosine triphosphatase pump. 15-17 Magnesium is also important because of its role in protein synthesis, vascular smooth muscle tone, activation of T cells, depolarization of myocardial cells and neurons, and production of intracellular second messengers, such as cyclic adenosine monophopshate. 15-17 Magnesium is primarily an intracellular cation; 99% is contained within the cells and less than 1% is found in extracellular fluid. 15,16 Serum magnesium exists in three forms: ionized (55% to 65%); protein-bound, primarily to albumin (30% to 40%); and complexed to anions, such as citrate, phosphate bicarbonate, lactate, or sulfate (4% to 6%). 15,16,18 Most total-body magnesium is found in bone, and homeostasis is regulated by a balance between renal excretion and intestinal absorption. 1 Recent veterinary research has found an association between altered total serum magnesium concentrations and increased morbidity, mortality, and prevalence of other electrolyte disorders in critically ill animals. 19 Most magnesium-related disorders are precipitated by conditions that deplete total-body magnesium stores. Magnesium deficiency can result from decreased intake, increased losses (primarily renal and gastrointestinal), and alterations in distribution, such as extracellular to intracellular fluid shifts. 1,16 Clinical signs associated with magnesium deficiency can include refractory hypokalemia and hypocalcemia, cardiac arrhythmias, muscle weakness, seizures, mental depression, hemolysis, anorexia, nausea, and ileus. 1,15,16 Reduction of total or ionized magnesium indicates hypomagnesemia and may warrant supplementation. However, a normal total or ionized serum magnesium concentration does not always imply normal total-body stores because only 1% or less of total-body magnesium is found in the extracellular compartment. 15,16 For people in whom magnesium deficiency is suspected but not documented, a magnesium loading test is recommended if renal function is normal. This test measures the body's retention of magnesium and therefore reflects the degree of deficiency. 15 A similar test has not been described in veterinary patients, so it is important to recognize patients at risk for magnesium disorders and understand that intervention may be warranted even with a normal serum magnesium Fluid and electrolyte therapy should be tailored to each individual patient and doses calculated as is done with the administration of any drug. RED, GREEN, PURPLE When blood samples are collected, blood tubes should be filled in the following order: first, red top (for serum collection); next, green top (containing citrate); and finally, purple top (containing EDTA). shutterstock.com/CNK02

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